We systematically searched the English-language literature for studies of the effects of inadequate housing on the development of TB, and the consequences thereof. The study summarized the effects of inadequate housing, in terms of affordability and quality, on infection with TB. We reviewed 26 studies published since 2011, identified by a search of four reliable electronic databases. All eligible studies emphasized the importance of housing affordability and quality in the prevention, early detection, treatment, and management of TB. This study, which defined inadequate housing as unaffordable housing of low quality, confirmed that compared with residents of adequate housing, residents of inadequate housing had a significantly higher risk of developing each of the eight steps in TB.
We found that both the affordability and quality aspects of inadequate housing were associated with TB exposure (TB bacilli inhalation), incidence, and treatment adherence. We also found that poor housing affordability was associated with almost all eight steps of TB development and its consequences. Homelessness, which was highly related to TB infection, is increasing worldwide. The risk of further TB spread and the development of drug-resistant TB are serious issues, particularly in terms of the accelerated rate of homelessness in low-income countries , where the numbers of new or recurrent cases are high.
Poor housing quality indicates that the indoor environment is inadequate, although there is no problem with housing affordability. Overcrowding, such as shared housing, is a typical risk factor for TB [2, 7, 8, 52]. Overcrowding directly affects TB exposure, transmission, incidence, and drug resistance, and is also associated with lack of sunlight, dampness, and improper ventilation. The WHO defines crowding as a lack of space within a “dwelling for living, sleeping, and normal family/household life”. It has found a dose-response relationship between housing and health outcomes. Crowding measures the relationship between the number of residents in a space and the amount of residential space (i.e., rooms or floor area) available . Studies that measure overcrowding should report the area of the living space and the number of persons per room instead of using arbitrary definitions.
Poor ventilation itself increases the risk of death regardless of socioeconomic status . Along with crowding, poor ventilation has been identified as having negative effects on health; ventilation regulations differ from country to country and by the space in the house and are not easy to quantify . Assessing ventilation with the presence of a window alone is inappropriate. Even if there is an operable window, it may not provide proper ventilation because of “outdoor temperature, noise, comfort, energy costs, the condition of windows or doors, or cultural and personal habits” . Improper ventilation, resulting from no windows, kaccha housing, living on the lower floors of high-rise buildings, and overcrowding, is associated with disease transmission and increases the incidence of TB by prolonging the time that TB bacilli droplets remain suspended in indoor air. The lower floors of high-rise buildings (most public housing) can lead to a higher incidence of TB due to a lack of sunlight and improper ventilation. Uncoated walls or leaking roofs increase dampness in housing and are associated with an increased incidence of TB.
The most important urgent medical intervention for TB is to begin treating active TB before transmission. Drug resistance and treatment completion are major challenges in TB eradication; the latter can be hindered by multiple factors and poses a high risk of resulting in drug-resistant TB. The main reason TB patients stop treatment is related to their economic conditions, not only their income but also the cost of completing treatment , including transportation to the treatment facility, food for essential nutrition during treatment, and housing. Housing affordability due to reduced income during TB treatment can be both a reason for, and a result of, not being able to adhere to treatment. Poor housing affordability decreases rates of TB detection  and treatment adherence [26, 28] and access [25, 26, 28–30, 32]. In particular, homelessness increases the risk of the recurrence of TB . The WHO has identified seven intervention categories for adherence to TB treatment: “supervising treatment (e.g., directly observed therapy), reminders and traces, incentives and enablers, patient education, digital technologies, staff education, and combinations of these interventions” . Housing is directly or indirectly related to all intervention categories affecting adherence to TB treatment.
The importance of housing affordability has been confirmed by intervention studies of housing support, particularly as regards homelessness, resulting in reduced hospital stays and follow-up times for TB patients [27, 29, 30, 32]. Public health interventions may differ according to the step of disease development, but TB is a complex set of processes, and thus a combined intervention approach is important. Specifically, patient education is recommended to reduce the risk of TB infection , and subsidies  or treatment methods requiring a shorter period  all help to increase the success rate of treatment. Integrated preventive interventions for community health workers and professional TB centers can reduce delays in TB treatment and expand access to TB treatment facilities .
There are many risk factors associated with TB other than housing. These risk factors were included in the final analysis along with housing and were controlled. None of the 26 eligible studies tested the interaction between housing and other factors to verify the role of effect modifiers. The housing factor and TB were deemed to be independent in the selected studies. In the final model of each paper, the factors with the greatest effects other than housing were smoking and substance use. The risk factors from the selected studies are detailed in a separate table (see Additional file 4: Table S4_2).
The study distribution and housing issues of interest varied depending on the economic level of the country. High-income countries may recognize that TB is more prevalent with certain types of housing, such as homelessness [26, 27, 30–33, 35] or public  and shared  housing. It may be difficult for low- and middle-income countries to perceive housing as a risk factor of TB, as the prevalence of TB in the general population is still high. According to a WHO report, the prevalence of TB is still 150–400 cases per 100,000 population in 30 countries with high burden, and more than 500 cases per 100,000 population in the Central African Republic, the Democratic People’s Republic of Korea, Lesotho, the Philippines, and South Africa . Furthermore, high-income countries may have a relatively sufficient budget and infrastructure to implement active strategies to end TB whereas in low- and middle-income countries, despite an increasing consensus that action to address social determinants of TB is necessary, practical ideas for such actions are scarce .
Strengths and limitations
This systematic review was a rigorous review of studies of various designs conducted in different countries. However, there are several potential limitations to this work. First, we may have missed some important articles, as we only included literature published between Jan 1, 2011 and Oct 25, 2020. Second, due to the linguistic limitations of the researchers, the articles were limited to English only. Third, a meta-analysis could not be performed because of the heterogeneity among populations and studies. For example, some studies defined homelessness as street homeless only, without specific explanation, while others defined homelessness as living on the streets, in shelters, or in health facilities. There were some differences in the definition of treatment completion or success, such as when more than 88% of the prescribed regimen was achieved or when a negative smear was confirmed after 4 months of treatment.
Nevertheless, this is the first systematic review to evaluate the effects of inadequate housing on TB and to categorize inadequate housing in terms of affordability and quality. We attempted to define the pathways leading to TB in terms of housing characteristics based on housing and health frameworks [13, 19] and frameworks of proximal risk factors for TB [7, 8]. We found that housing of inadequate quality increased the risk of TB development due to TB exposure and transmission, rendering it difficult to adhere to treatment and increasing the risk of multidrug-resistant TB. Without affordable housing, detection of TB infection, exposure and incidence, and of successful treatment completion, which affect all eight steps of TB detection, treatment and prevention are difficult.